The object of the invention is an austenitic stainless steel product, such as a strip, plate, sheet, bar or wire, manufactured from austenitic stainless steel.
The object of the invention is also a method for manufacturing a very strong austenitic stainless steel product, selected from the group consisting of a plate, a steel strip, a steel bar and steel wire wherein the steel product is cold-rolled and heat-treated.
The invention relates generally to austenitic stainless steels and to products manufactured from them, such as strips, plates, bars, wire, et cetera. The invention also relates to a method for elevating the strength of an austenitic stainless steel product by decreasing the grain size, while nevertheless preserving the good toughness properties characteristic to austenitic stainless steel. Stainless steels are usually divided into four main types: austenitic, ferritic, austenitic-ferritic (duplex) and martensitic stainless steels. Austenitic stainless steels, which typically contain at least 16% Cr (chromium) and typically 8-14% Ni (nickel), are well suited for applications in which high tensile strength, formability, weldability and good corrosion resistance are required.
Austenitic stainless steels can be classified into a number of sub-types, of which the two most important main types are AISI 304 and AISI 316 as well as the corresponding low-carbon steel grades AISI 304L and AISI 316L. An important difference in these is their chemical composition with respect to molybdenum. AISI 316 and AISI 316L contain approx. 2-3 percent molybdenum by weight, whereas molybdenum is not generally deliberately added to AISI 304 and AISI 304L steels. More particularly, resistance to pitting corrosion is essentially better with AISI 316 and AISI 316L steels than with AISI 304 and AISI 304L steels. The nickel content in the aforementioned Cr—Ni—Mo steels is typically between 10-14% and in Cr—Ni steels typically between 8-12%.
Most of the world's stainless steel is manufactured from raw steel and ferrochromium with the AOD method. The charge is smelted in an arc furnace (smelting unit) and the melt is treated in a converter (metallurgical unit). Decarburizing in the AOD converter occurs by blowing a mixture of oxygen and an inert gas (argon) into the melt. Decarburizing of the melt occurs in stages in such a manner that when reducing the carbon content the proportion of inert argon increases during the blowing. The treatment phases comprise, in addition to decarburizing, slag reduction with silicon, desulphurization and alloying. After the AOD process the melt is poured into a casting ladle, in which ladle treatment is performed. The purpose of this is to finish the composition of the steel and to adjust the temperature of the melt for the casting. The liquid steel is cast with a continuous casting machine.
The slabs made with continuous casting are hot-rolled into strips, which are heat-treated and pickled on continuous-action lines. The pickled hot strip is generally still cold-rolled to make it thinner and finally annealed and pickled. Acid treatment removes the oxide scale produced on the surface of the steel in heat treatment.
The surface of the pickled strip is matt-like. The surface of the strip can still be slightly skin-pass rolled, brushed or polished for achieving a different surface appearance.
In addition to strip, also other products, such as plate products, sheets, wire, bars, et cetera, are manufactured from stainless steel in a manner that is per se known in the art. Plate product manufacture, sheet manufacture, bar manufacture and wire manufacture have their own process phases, differing from strip manufacturing, which phases are per se known to a person skilled in the art, and they are not described in more detail in this context.
One disadvantageous property of austenitic stainless steels is considered to be their low strength, which has limited their use as a structural material. The yield strength (Rp0.2) of austenitic stainless steels at room temperature is usually between 230-300 MPa, whereas the yield strength of high-strength low-alloy steels (not stainless steels) can be twice or even quadruple that. The strength of stainless steel can be increased by cold-rolling, but when using it a significant part of the formability of the material is lost. Nowadays in the industrial manufacturing of stainless steels a high temperature, which is e.g. in the region of 1050° C., is used as the annealing temperature of the heat treatment following cold-rolling. In this case a grain size is reached, which is approx. 20 μm, as a result of the heat treatment. From this follows a rather low yield strength, which is typically less than 300 MPa.
One desired method of increasing the strength of stainless steels is to metallurgically reduce the average grain size of the metal. The conventional grain size is approx. 20 μm, but by reducing the grain size e.g. to approx. one-tenth, the yield strength could be doubled. The reason is that it has been proven that the yield strength of metals increases linearly as a function of the inverse value of the square root of the grain size. It has been possible to manufacture these types of so-called “ultra fine grain” (UFG) steels possessing a small grain size in laboratory conditions, but a viable and economic manufacturing process has not been found for these steels.
Solutions are known in which austenitic stainless steel is cold-rolled, in which case, especially when using steel grades containing unstable austenite, most of it changes in the cold-forming into extremely hard martensite. After this, heat treatment is performed on the steel to form a microstructure, which contains mainly very fine grained austenite produced with the so-called reversion mechanism from martensite and often also non-recrystallized austenite in the cold-formed state. Known from publication EP1899490 is a strip consisting of austenitic stainless steel, said strip having a certain chemical composition, and in the manufacturing of which alpha martensite (α′-martensite) produced in the shape deformation forms 50-90 percent by volume, the reduction ratio in the cold-rolling being 55-85%. According to the publication it is essential that the reduction ratio is sufficiently high for the amount of martensite forming to be sufficient for producing the required properties. In the publication in question, reversion annealing is then used to change the martensite into very fine-structured austenite. The martensite produced in cold-forming, however, has properties that make it undesirable. It is hard and brittle, in which case, especially when cold-forming a steel strip by rolling, the properties of martensite set limits to the reduction ratio and, on the other hand, to the durability of the rolling equipment used.
One aim of the invention is to achieve a completely new type of austenitic stainless steel product, by means of which the weaknesses of current stainless steel products can be improved. Another aim of the invention is to achieve a method for elevating the strength of an austenitic stainless steel product by decreasing its grain size while at the same time preserving well the good toughness characteristic to austenitic stainless steel. Yet another aim is to achieve a solution, by means of which the drawbacks of prior art are avoided.